Method for assembling signatures

ABSTRACT

A method for electronic assembly of signatures where individual printed pages are assembled to form a signature. Contents of the printed pages to be assembled are described in a page description language. Contone Contone maps in which the printed pages are described pixel-by-pixel and line-by-line in the form of gray scale values are generated for the individual printed pages by interpretation of page descriptions in at least one interpreter. The contone maps of the individual printed pages are assembled to form an overall contone map in a raster generator such that lines of the overall contone map of the signature are composed from individual lines of the contone maps of the printed pages taking predetermined parameters into consideration. The overall contone map of the signature is converted into a screened bit map in the raster generator. The signature is recorded pixel-by-pixel and line-by-line on a recording medium based on the bit map.

BACKGROUND OF THE INVENTION

The invention is in the field of electronic reproduction technology andis directed to a method for the electronic assembly of signatures in araster generator from a plurality of printed pages that are present ashigh-resolution contone map.

In reproduction technology, printer's copies that contain all elementsto be printed such as texts, graphics and images are generated forprinted pages. A separate printer's copy that contains all elements thatare printed in the respective color is generated for each ink inchromatic printing. For four-color printing, these are the inks cyan,magenta, yellow and black (C, M, Y, K). The printer's copies separatedaccording to inks are also called color separations. The printer'scopies are usually screened and exposed on films with high resolutionthat are then further-processed for the production of printing forms(printing plates, printing cylinders). Alternatively, the printer'scopies can also be directly exposed on printing plates in specialrecorders. For checking the content and the colors of the printed pages,printer's copies are exposed in proof recorders with a recording processthat simulates the printing process in a chromatic output.

FIG. 2 shows the work sequence that was previously mainly employed inthe prior art in the exposure of printer's copies for printed pages thathad been generated in the page description language PostScript.PostScript data 1 are supplied to a raster image processor (RIP) (2)that can be a computer specifically optimized for this job or a programon a standard computer. PostScript data 1 for every color separation arenormally generated in a pre-process for every color separation of aprinted page and are forwarded to the RIP (2) (separated PostScript).Alternatively, a chromatic printed page can also be generated in asingle PostScript dataset (composite PostScript). The case of separatedPostScript data 1 shall be explained in greater detail below.

In a first step, the PostScript data 1 are analyzed in an interpreter 3and resolved into a sequence of simple graphic objects. For thatpurpose, the printer's copy is divided into horizontal strips (bands)that are successively processed. FIG. 3 shows a band excerpt 9 with afew objects generated by the interpreter. The band excerpt 9 is dividedinto recording pixels 10. In the example of FIG. 3, the band excerpt is8 pixels high, numbered from 0 to 7, and 32 pixels wide, numbered from 0to 31. The resolution can be symmetrical (the same in horizontal andvertical direction) or asymmetrical, for example twice as greathorizontally as vertically. The objects A through E (11,12,13,14,15)describe sub-segments of text, graphics or image elements that fallwithin the band excerpt 9.

The interpreter outputs the objects A through E (11,12,13,14,15) in adata format that is referred to as display list 4 (FIG. 2). For eachobject, the data format describes its geometrical shape and the grayscale value with which it is filled. The objects A through E(11,12,13,14,15) appear successively in the display list 4 in thesequence in which the corresponding page elements are described in thePostScript data. Objects that appear later in the display list (4) canthereby partly or entirely cover objects that appeared earlier in thedisplay list 4. In the example of FIG. 3, the object A 11 is partlycovered by the object B 12. Likewise, the objects D 14 and E 15 coverthe object C.

In a further step in the RIP 2, the display list 4 is supplied to araster generator 5 that successively converts the objects of the displaylist 4 into surfaces filled with raster points and enters them into abit map memory 7 as bit map data 6. The raster point size is therebyvaried dependent on the gray scale value of the object in the displaylist 4. The bit map data 6 of objects that appear later in the displaylist 4 respectively overwrite the corresponding areas of the bit mapmemory 7. After all objects of a band have been rastered by the rastergenerator 5 and written into the bit map memory 7, the content of thebit map memory 7 is forwarded as control signal values to the recorder 8and exposed thereat.

As a rule, it is not only one but a plurality of printed pages at oncethat are printed with a printing plate, these being arranged such thatthe area of the printing plate is used well and such that, after foldingand cutting the printed paper sheet, the printed pages yield a brochure,a leaflet or the like. For that purpose, the printed pages that are tobe printed on the paper sheet in the same printing event are combined ina signature. The arrangement of the printed pages in a signature isreferred to as an imposition pattern. As an example, FIGS. 4a and 4bshow the imposition patterns for a brochure with 16 printed pages. FIG.4a shows a printing plate 16 on which a signature 17 is arranged. Thesignature 17 unites all elements to be printed, i.e. the printed pages18 and auxiliary elements such as register marks 19, fold and cut marks20 and print control strips 21. These auxiliary elements serve forquality control during printing and for simplifying thefurther-processing (folding, cutting, binding). The numbers in theprinted pages 18 in the imposition pattern identify which page of thebrochure is printed at which location of the signature. Numbers that areupside down identify pages that are printed upside down. FIG. 4a showsthe pattern that is printed on the recto of the paper sheet (obverse)and FIG. 4b shows the pattern that is printed on the verso of the samepaper sheet (reverse). With the imposition pattern of FIGS. 4a and 4b,the pages are arranged continuously in the brochure after the printingof both sides of the paper sheet and after the folding and cutting.

In the prior art, there are two essential methods for assembling pagesthat are present as PostScript data to form a signature, manual assemblyand electronic assembly of the PostScript data. In manual assembly, thePostScript data of all pages are first interpreted in a RIP, and thecolor separation films of the pages are exposed on a recorder, as shownin FIG. 2. 64 color separation films thus arise in the example of thebrochure with 16 pages (16 pages×4 inks). Eight printing plates eachhaving Eight pages are required for the printing (4 respective inks forthe obverse and the reverse). Given manual assembly of the 8 signatures,8 color separation films of the pages per signature must be glued onto atransparent film having the size of the signature according to thearrangement of the imposition pattern, for example the cyan films ofpages 1,4,5,8,9,12,13,16 according to the imposition pattern of FIG. 4a.The films of the other inks are likewise respectively glued onto a largeassembly film according to the same pattern. One proceeds accordinglyfor the color separation films of pages 2,3,6,7,10,11,14,15 butaccording to the imposition pattern of FIG. 4b. The printing plates arethen produced by contact exposure with the assembled films in aphotographic process. Manual assembly work must be very carefully andexactly carried out since the signatures of the individual inks must becongruent so that no color fringes occur at the image or text edges inthe final print and so that the sharpness of the printed images is notdeteriorated. It is obvious that manual assembly of the signatures isextremely work-intensive, time-consuming and susceptible to error aswell.

The production of a PostScript description of the entire signature isstandard as an electronic assembly method for signatures in the priorart. For that purpose, the PostScript data of all pages are collected ina preliminary process on a computer (server), and, when the arecompletely present, are linked with PostScript data for the registermarks, fold/cut mark and print control strips to form an extensive fileof PostScript data for the entire signature. A respective PostScriptfile is usually generated for each ink and for each side of the papersheet. In the example of the 16-page brochure, 8 PostScript files thusarise for the 8 signatures (respectively 4 inks for the obverse and thereverse). These PostScript files are then interpreted in the RIP,screened and exposed in a large-format recorder on films having the sizeof the signatures or directly on printing plates.

It is also standard to mix both methods, for example the PostScriptassembly of respectively half of a signature and the manual assembly ofthe two halves to form an entire signature. On the one hand, a recorder(expensive) with a very large exposure format is not required; on theother hand, however, the manual assembly work is greatly simplified.

PostScript assembly of the signatures also has disadvantages. First, thePostScript data for a signature can be extremely extensive and complex,so that a very high-performance and, thus, expensive computer isrequired in the RIP for the interpretation. Since the individual printedpages are often produced by used programs from different manufacturers(text processing, graphic design and image processing programs), it canoccur that the PostScript data of some pages cannot be correctlyprocessed by the interpreter in the RIP or that the RIP even freezes upduring the exposing. This is the case when the manufacturers of the userprograms have not exactly adhered to the rules of the PostScript pagedescription language. This is more critical in exposing signatures thanwhen exposing single pages since the signature exposing can only bestarted when all pages are finished. This, however, often occurs onlyshortly before that start of printing, so that there is no more time tosearch for the error.

There would also fundamentally be the possibility of having theindividual pages interpreted and screened by the RIP and by notimmediately forwarding the bit map data thereby generated to therecorder for exposure but, for example, to intermediately store them ona disk storage. The bit map data of all pages could then be operated ina computer (server) according to the imposition pattern to form a bitmap dataset for the entire signature and then be subsequently exposed.Such a system is disclosed in published application DE 40 26 321 A1,whereby images are screened and stored as compressed bit map data. Thissolution, however, is not practical for higher exposure resolutions asrequired for the recording of printer's copies since the memory for thesignature bit map becomes extremely large and expensive. A memoryrequirement of 3109 Mbytes per signature derives for a printing platehaving the size 70 cm×100 cm and a resolution of 2666 pixels/cmhorizontally (6772 dpi; dpi=dots per inch) and 1333 lines/cm vertically(3383 dpi). A storage space of 24876 Mbytes is then required for the 8signatures of the 16-page brochure. Hard disks are also eliminated as astorage medium since they cannot read out the bit map data with therequired recorder data rate of 100 to 200 Mbits/s.

Due to the high memory requirement for the finished bit map of asignature, the bit map of a signature cannot be intermediately stored inthe previous procedure for the assembly of PostScript data. When thesame signature is to be exposed again, for example because the filmexposed first or the printing plate was damaged, the entire processingsequence from the interpretation of the Post Script data up to theexposure must be run through again. This costs additional time andoccupies the RIP that could already process a new signature during thistime. For the same reason, the additional exposure of the signature on aproof output device in the previous procedure again requires the entirerun of the PostScript data through the RIP and therefore costsunnecessary time. This is a further disadvantage of the assembly ofPostScript data in the prior art.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to avoid theaforementioned disadvantages and disclose a method with which compressedand overlap-free contone maps (delta lists) of the printed pages aregenerated with little memory outlay and are decompressed at high speedduring the rastering and exposure and assembled to form a signature.

This enables the cost-beneficial intermediate storing of the printedpages and the exposing of signature films and proof outputs of thestored contone maps without having to interpret the PostScript data aneweach time.

The compression of the contone maps enables the conversion in screenedbit map data at high speed and without intermediately storing the entiresignature and enables the exposing without a start/stop mode of therecorder.

This object is achieved by employing a new data format for contone mapsthat is also referred to as a delta list. The PostScript pagedescriptions of the individual printed pages are first converted intocontone maps by an interpreter and are subsequently compiledline-by-line to an overall contone map of the signature during thescreening and exposing.

The invention is described in greater detail below with reference toFIGS. 1 through 7.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a printed page with text, graphics and imageelements (prior art);

FIG. 2 shows the work execution in the exposing of PostScript dataaccording to the prior art;

FIG. 3 shows an excerpt from a band with objects that the interpretergenerates (prior art);

FIGS. 4a-4b are an example of imposition patterns of a 16-page brochure(prior art);

FIG. 5 shows the work execution given the exposing of PostScript datawith the generation and further-processing of the delta list;

FIG. 6 shows the subdivision of a printer's copy into bands and zones;and

FIGS. 7A-7B show the assembly of delta lists to form a signature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Generalities

German patent application of the assignee "Method For Generating aContone Map" corresponding to U.S. Ser. No. 08/737,892 filed Feb. 19,1997, pending discloses the generation of a contone map (delta list) indetail. This shall therefore be only explained to such an extent here asrequired for an understanding of the inventive method for assembling asignature on the basis of contone maps (delta lists) of the individualprinted pages.

A contone map describes a reproducible printer's copy in the form ofgray scale values in which a gray scale value is allocated to eachpixel. The contone map is generated from the page description data(PostScript data) of the printed page to be reproduced. The gray scalevalues of the contone map can be directly employed for the control ofthe recorder when the recording process can reproduce continuous tonalvalues such as, for example, a proof output means. For recordingprocesses that can only reproduce two tonal values (white or,respectively, black), the gray scale values are converted into rasterpoints before the recording in a raster generator that precedes therecorder, the gray scale values being simulated for the eye therewith.In the recorder, the printers copies are exposed onto the recordingmaterial pixel-by-pixel and line-by-line with at least one exposurebeam. During the exposure, control signal values determine which pixelsare exposed as parts of the raster points or not in that the controlsignal values correspondingly switch the exposure beam on and off.

For editing the delta list, the superimpositions of the objects in thedisplay list (FIG. 3) are suitably eliminated and the data aresubsequently compressed as highly as possible. The delta list is free ofoverlap because it only contains objects that adjoin one another and donot overlap. There is only one gray scale value for each pixel in thedelta list. A compromise between a high compression factor, a fastcompression and, above all, a very fast decompression must be found inthe selection of the compression method.

The delta list essentially contains simple graphic objects and rasterinformation that can be converted into bit map data and output by araster generator keeping pace with the recorder speed.

The generation of the delta list and the screening can be implementedwith different resolutions. An advantageous version is the calculationof the objects in the delta list with 666.5 pixels/cm (1693 dpi) and thescreening of the gray scale values with 1333 pixels/cm (3386 dpi). Thescreening can also ensue asymmetrically, for example with 2666 pixels/cm(6772 dpi) in line direction and 1333 pixels/cm (3386 dpi) perpendicularto the line direction.

The format of the delta list is byte-oriented. Each byte is a commandthat is followed by data bytes in some instances. The coding of thecommands is selected such that an optimally great compression of thedata is achieved. General information, for example the length of thedelta list and the length of a scan line, are located at the start ofevery delta list. The delta list also contains information about thescreening method according to which the objects are to be converted intobit maps by the raster generator.

FIG. 5 shows an improved work sequence for the interpretation andexposure of PostScript data, whereby a contone map in the data format ofthe delta list is generated. The PostScript data 1 that describe thecontent of the printer's copy are supplied to the RIP (2) where, in afirst step, they are analyzed by the interpreter 3 and converted into adisplay list 4, as was already explained above. In a second step, adelta list generator 22 generates the overlap-free contone map of thedelta list (23) from the display list, and this is stored, for example,on a disk storage (24). When individual printed pages are to be exposed,the stored delta lists of the printer's copies, for example the variouscolor separations of a printed page, are successively called from thedisk storage 24 at a later time, are converted into bit maps 6 by theraster generator 5 and are exposed in the recorder 8. The screening ofthe delta list ensues keeping pace with the recorder speed.

Since very different page contents with different properties withrespect to the compression can occur in different parts of a printedpage, the printed page is divided into horizontal strips (bands) in thegeneration of the delta list and these bands are further subdivided intosuccessive sections (zones). Respectively optimized compression methodscan then be applied in the bands and zones.

FIG. 6 shows the division of a printer's copy 25 into bands 26 and zones27. The height of the bands and the width of the zones is arbitrary;however, it is advantageous for the processing when the bands are all ofthe same height and the zones are all of the same width. It is alsoadvantageous when the band height and the zone width are powers of 2.

Since large parts of the information on a printed page are oftencomposed of few different gray scale values, for example only ofblack/white information (text), gray scale values in the delta list areencoded with different numbers of bits, for example 1-bit gray scalevalue for black/white information and 8 bits per gray scale value forcontone information. This measure likewise contributes to thecompression of the delta list.

The compression of the data in the inventive data format of the deltalist is based on the run length method that is modified for the specificdemands. Command bytes that can be accompanied by a run length and/orone or more gray scale values exist in the data stream. The compressionalso considers repetitions of the entire content of a zone inY-direction (X-direction=principal scanning direction;Y-direction=secondary scanning direction). A few delta list commands andtheir encoding that are important for understanding the generation ofthe delta list are explained by way of example in the following table.

    __________________________________________________________________________    Start of a new band:     ##STR1##    Start of a new line in the band:     ##STR2##    Start of a new zone in the line:     ##STR3##    Y-cmpr = number of repetitions in Y-direction    bits = number of bits per gray scale value (1, 8, 12)    Selection of a screening method:     ##STR4##    index = number of the screening method for the following gray scale         values    Short run length:     ##STR5##    The gray scale value is repeated (nnnnnn+1) times.    Long run length:     ##STR6##    The gray scale value is repeated ( nnnn!×256+ kkkk kkkk!+1) times.    Uncompressed data:     ##STR7##    (nnnnn+1) uncompressed gray scale values follow.    __________________________________________________________________________

The first byte or, respectively, the first bits in the first byte ofeach command are an indication of the command that is involved and ofhow many bytes with parameters for the command follow. This structureassures that each command can be unambiguously recognized and correctlyinterpreted in the decoding of the delta list.

Each new band is initiated with the command LHD₋₋ BAND and each new linewithin the band is initiated with the command LHD₋₋ START. The commandLHD₋₋ ZONE wherein the number of lines over which the content of thiszone repeats in Y-direction is encoded with the parameter "Y-cmpr"stands at the start of every zone in the line. The parameter "bits"indicates the number of bits with which the gray scale values areencoded within the zone, for example 1 bit for black/white information,8 bits for contone information with normal graduation (256 steps) and 12bits for contone information with finer graduation (4096 steps).

A screening method that is identified by the parameter "index" isselected with the command LHD₋₋ SCREEN. The raster generator shouldscreen all following gray scale values in the delta list with theselected screening method until a new screening method is selectedagain. The parameters of the screening method such a screen width,screen angle, raster dot shape are stored under the number "index" inthe raster generator or they are attached to the generated delta listwith further delta list commands.

A run length of repeating gray scale values within a zone is describedwith the commands LHD₋₋ REPEATS or LHD₋₋ REPEAT. In the command LHD₋₋REPEATS, a 6-bit binary number nnnnnn! in the first byte encodes a runlength between 1 and 64; a run length between 1 and 4096 is encoded inthe command LHD₋₋ REPEAT by a 12-bit binary number ( nnnn! in the firstbyte and kkkk kkkk! in the second byte). The last byte of this commandrespectively indicates the gray scale value that should be repeated.

When successive gray scale values in the line are not the same and cantherefore not be compressed with a run length, such a sequence isdescribed with command LHD₋₋ UCDATA. A 5-bit binary number nnnnn! in thefirst byte indicates how many uncompressed gray scale values follow.

In the generation of the delta list, the lines of a band are processedfrom top to bottom and the zones of a line are processed from left toright. The generated commands and run lengths are thereby joined to oneanother tightly packed, i.e. nothing is entered in the delta list forthe zones for which no run lengths are generated. As a result of thecode for the compression in Y-direction in the command LHD₋₋ ZONE, theraster generator can decode the delta list such that the run lengths areagain allocated to the correct zones.

The Assembling of Delta Lists

It was explained above that individual printed pages in the delta listformat can be directed supplied to the raster generator 5, decompressedand screened thereat and forwarded to the recorder for exposing. When,however, entire signatures with a plurality of printed pages areexposed, then the delta lists of all printed pages and of all auxiliaryelements (register marks, etc.) in a memory are made available to theraster generator in the inventive method. In addition, a data structurethat defines the paper sheet (sheet definition) is deposited in thememory, and a data structure that defines the printed page or,respectively, the auxiliary element (delta definition) is deposited inthe memory for each printed page and each auxiliary element. The sheetdefinition contains all information needed for the recording of thesignature, such as

dimensions of the signature

position on the exposure surface of the recorder

resolution of the RIP

resolution of the recorder

positive/negative exposure

identification of the recording material

information for punching fitting holes

identifier for the identification of the signature.

The delta definitions of the printed pages or, respectively, auxiliaryelements contain information such as

dimensions of the printed page or, respectively, auxiliary element

position on the signature

reference to the identifier of the appertaining signature

reference to the appertaining delta list

identifier for the identification of the delta definition.

The execution of the assembling in the raster generator 5 during thescreening and exposure is now explained with reference to the example ofa simplified imposition pattern (without auxiliary elements), as shownin FIG. 7a. The raster generator assembles the signature line by lineand then immediately screens and exposes each assembled line. Whichprinted pages and auxiliary elements contribute to the structure of theline for a line 28 to be exposed is determined from the particulars inthe signature definition and in the delta definitions. These are theprinted pages 12, 5, 8, 9 for the line 28. Which line segments from theappertaining delta lists must be inserted into the line to be assembledis likewise determined from the position particulars in the deltadefinitions. The corresponding excerpts are fetched from the deltalists, decompressed and inserted into the current line 28. FIG. 7b showsthe structure of the line 28 again. A white line section (29) (i.e. aline section that is not to be exposed) from printed page 12 is insertedfrom the left edge up to the beginning of the line segment 30. The linesegments (32, 34, 36) from the delta lists of the remaining pages 5, 8,9 are inserted in conformity with their position in the line 28, and theinterspaces are filled with the white sections 31, 33, 35, 37. Theassembled line 28 is subsequently screened and forwarded to the recorderfor exposure. The next line is then assembled, etc.

The electronic assembly of signatures is implemented in this way duringthe screening and exposing. As a result thereof, it is not necessary tointermediately store the assembled, whole signatures before thescreening and exposing. The memory requirement is additionally limiteddue to the assembling on the basis of compressed delta lists. All saidproblems in the interpretation of extremely large and complex PostScriptfiles are also avoided by employing delta lists instead of PostScriptdata for the assembling.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that our wish is toinclude within the claims of the patent warranted hereon all suchchanges and modifications as reasonably come within our contribution tothe art.

We claim as our invention:
 1. A method for electronic assembly ofsignatures, wherein individual printed pages are assembled to form asignature, comprising the steps of:describing contents of the printedpages to be assembled in a page description language; generating contonemaps in which the printed pages are described pixel-by-pixel andline-by-line in the form of gray scale values for the individual printedpages by interpretation of page descriptions in at least oneinterpreter; assembling the contone maps of the individual printed pagesto form an overall contone map in a raster generator such that lines ofthe overall contone map of the signature are composed from individuallines of the contone maps of the printed pages taking predeterminedparameters into consideration; converting the overall contone map of thesignature into a screened bit map in the raster generator; and recordingthe signature pixel-by-pixel and line-by-line on a recording materialbased on the bit map.
 2. The method according to claim 1 wherein thepredetermined parameters in the raster generator are a definition of thesignature and a definition of the contone map for each printed page tobe assembled;the definition of the signature contains a signatureidentifier and information required for the assembly of the signature;and the definition of the contone map contains a corresponding contonemap identifier and further information for assembly.
 3. The methodaccording to claim 1 wherein a plurality of signature definitionscorresponding to a plurality of color separations are predetermined inthe raster generator for recording color separations.
 4. The methodaccording to claim 1 wherein the overall contone map of the signature isnot intermediately stored.
 5. The method according to claim 1 whereincontone maps of auxiliary elements selected from the group consisting ofregister marks, fold marks, cut marks, and print control strips areassembled with the contone maps of the printed pages to form the overallcontone map of the signature.
 6. The method according to claim 1wherein, for generating a contone map of a printed page:a programmedpage description of a content of the printed page, composed of text,graphic and image information, is processed by an interpreter and a listof graphic objects is generated; the objects are superimposed accordingto their position on the printed page; pixels are generated for theobjects; and the pixels are combined to form the contone map.
 7. Themethod according to claim 1 wherein the contone map is data-compressedaccording to a run length coding.
 8. The method according to claim 1wherein the contone map is data-compressed by reducing a number of bitsper gray scale value.
 9. The method according to claim 1 wherein thecontone map is data-compressed by difference coding between gray scalevalues of neighboring pixels.
 10. A method for electronic assembly ofsignatures, wherein individual printed pages are assembled to form asignature, comprising the steps of:describing contents of the printedpages to be assembled in a page description language; generating contonemaps in which the printed pages are described in the form of gray scalevalues for the individual printed pages by interpretation of pagedescriptions in at least one interpreter; assembling the contone maps ofthe individual printed pages to form an overall contone map in a rastergenerator such that lines of the overall contone map of the signatureare composed from individual lines of the contone maps of the printedpages; converting the overall contone map of the signature into ascreened bit map in the raster generator; and recording the signature ona recording material based on the bit map.